UPD: Never mind, that's the wrong answer as it's about HL7. Sorry about that.

There are two ways to do that:

1. Use EnsLib.MsgRouter.VDocRoutingEngine, it has Validation setting which is a string specifying types of validation to perform. Set to 1 to block documents that don't pass default validation. Default validation checks for DocType assignment and successful BuildMapStatus. This is equivalent to 'dm': 'd' - require DocType 'm' - require successful BuildMap status. Set to 0 to skip validation. In addition configure Bad Message Handler setting to handle messages which fail validation. I'm not sure it your messages would fail BuildMap validation, but that's the easiest option to implement.
2. Create a subclass of EnsLib.MsgRouter.RoutingEngine and implement OnValidate method to provide custom validation. You'll also need to add Validation property to settings, same as EnsLib.MsgRouter.VDocRoutingEngine  does. In OnValidate method you can perform any checks you like.

I would recommend opening a WRC for that one.

I don't think dynamic return value is possible.

As a workaround you can write the desired destination value in context and redefine doOneAction in your custom router engine to replace i.e. @destino value with a value from a context before calling ##super.

Use System Default Settings?

Use the ^ZMIRROR Routine, you need NotifyBecomePrimary callback.

Here's the code to create task to purge messages in all interoperability namespaces:

set sc = ##class(%SYS.Namespace).ListAll(.result) 
kill result("%SYS"), result("HSCUSTOM"), result("HSLIB"), result("HSSYS"), result("REPO") 
while 1 {set ns = $o(result($g(ns))) quit:ns="" continue:$e(ns,1,2)="^^" set $namespace = ns continue:'##class(%Dictionary.ClassDefinition).%ExistsId("Ens.MessageHeader") set task=##class(%SYS.Task).%New(),task.Name = "Purge old Interoperability data in " _ $namespace,task.NameSpace=$Namespace,task.TimePeriod=0,task.TimePeriodEvery=1,task.DailyFrequency=0,task.DailyFrequencyTime="",task.DailyIncrement="",task.DailyStartTime = 3600,task.DailyEndTime = "",task.StartDate = $p($H,",",1)+1,task.Priority = 2,task.Expires = 0,taskdef = ##class(Ens.Util.Tasks.Purge).%New(),taskdef.BodiesToo = 1,taskdef.KeepIntegrity = 1,taskdef.NumberOfDaysToKeep = 1,taskdef.TypesToPurge = "all",sc = task.AssignSettings(taskdef),task.TaskClass=$classname(taskdef),sc = task.%Save()}

Here's how to fix that.

1. Create Inbound adapter which extends default inbound adapter and exposes DeleteFromServer setting:

Class Test.InboundAdapter Extends EnsLib.File.InboundAdapter
{
Parameter SETTINGS = "DeleteFromServer:Basic";
}

2. Create Passthrough Service, which uses your custom adapter:

Class Test.PassthroughService Extends EnsLib.File.PassthroughService
{
Parameter ADAPTER = "Test.InboundAdapter";
}

3. Use your class (2) when you create a new BS, it will have DeleteFromServer property:

I filed an enhancement request, please use DP-422980 as an identifier if you would contact WRC on this topic.